# Converting CM to Units
**URL:** https://discourse.psychopy.org/t/converting-cm-to-units/24891
**Category:** Builder
**Created:** [September 9, 2021, 11:28am UTC](https://discourse.psychopy.org/t/converting-cm-to-units/24891 "2021-09-09T11:28:25Z")
**Posts on this page:** 3
**Page:** 1
### Author:  [@Avishai\_D](https://discourse.psychopy.org/u/Avishai_D)
#### Post date: [September 9, 2021, 11:28am UTC](https://discourse.psychopy.org/t/converting-cm-to-units/24891/1 "2021-09-09T11:28:25Z")
Hi community,
I have a 1920 \* 1080 screen and I have trouble converting a 7.5 cm circle (H and W) to **norm** units.
What calculation do I need to make to find the right amount of units?
Thanks
---
### Author:  [@TParsons](https://discourse.psychopy.org/u/TParsons)
#### Post date: [September 11, 2021, 1:56pm UTC](https://discourse.psychopy.org/t/converting-cm-to-units/24891/2 "2021-09-11T13:56:35Z")
You can set the units of the circle to `cm` even if the rest of your experiment is in `norm` - PsychoPy will handle the conversion for you.
To do the conversion by hand, you need the physical size of the screen. Width in pixels / width in cm gives you the horizontal pixel/cm ratio, then you can do the same with heights to get the vertical ratio. Multiplying the appropriate ratio by the desired number of centimeters gives the necessary number of pixels, which you can then divide by the width or height of the screen in pixels to get the value in `norm` units.
All that is a bit of a faff, so wherever possible it’s safer to let PsychoPy handle it!
---
### Author:  [@Avishai\_D](https://discourse.psychopy.org/u/Avishai_D)
#### Post date: [September 11, 2021, 4:40pm UTC](https://discourse.psychopy.org/t/converting-cm-to-units/24891/3 "2021-09-11T16:40:02Z")
> [@TParsons](#):
>
> conversion by hand, you need the physical size of the screen. Width in pixels / width in cm gives you the horizontal pixel/cm ratio, then you can do the same with heights to get the vertical ratio. Multiplying the appropriate ratio by the desired number of centimeters gives the necessary number of pixels, which you can then divide by the width or height of the screen in pixels to get the value in `norm` units.
>
> All that is a bit of a faff, so wherever possible it’s safer to let PsychoPy handle it!
Thanks alot! it worked well